Tracking the bacterial dynamics in three dimensions

We present experimental results on the tracking of swimming {\it Escherichia coli} cells using a novel 3D particle tracking method. {\it E.coli} is a single cell organism, it is about 1-3 $\mu$m in size. Under favorable condition, it can run at a speed of ~30 times of its body length in one second. Swimming {\it E.coli} cells provide us an unique opportunity to probe the transport properties of a nonequilibrium system, where it consists of self propelled objects. In our experiments, a wildtype {\it E.coli} (RP437) is used for its known motile behavior. The cells are made fluorescent by the insertion of a plasmid that expresses GFP (Green Fluorescent Proteins) constitutively. As a result, the cells emit fluorescent light all the time. The cells are placed in a 5mm diameter and 1.5 mm depth well before filming. The 3D trajectories of multiple swimming {\it E.coli} cells are obtained for the first time using a novel defocused particle tracking technique. Various types of locomotion of bacteria are observed, running, tumbling, and wobbling. Using the track data, we evaluated the diffusion coefficient of the swimming cells. It demonstrated a ballistic behavior at the early time, and gradually develop into a random walk in later time. The Diffusion coefficient is about $10^{3}$ orders of magnitude larger than a system with nonmotile microorganisms of similar size.